LONDON, U.K. - In a damning revelation, it has now come to light that the Manchester Arena suicide bomber, who targeted young people at the end of a concert by U.S. singer Ariana Grande on May 22, could have been prevented from committing the atrocity.

According to a review conducted by author, David Anderson QC, the Manchester Arena bomber had been a "subject of interest" and opportunities to stop him were missed.

The review claimed that it was conceivable that the bomber Salman Abedi's attack, which killed 22 people, could have been avoided had "cards fallen differently.”

Anderson noted in his review that it was "unknowable" whether reopening investigations into Abedi would have thwarted his plans, and adding, "MI5 assesses that it would not."

According to the Greater Manchester Police, its officers would "never stop learning.”

The reviews were conducted internally by counter-terror police and MI5 after the Manchester bombing and three terror attacks in London this year.

Anderson is said to have carried out an independent assessment of the findings of these major internal reviews which remain largely secret.

The review by Anderson pointed out that the Manchester bomber Salman Abedi was a "Subject of Interest" for MI5 and that he had been investigated between January and July 2014, and then again in October 2015.

Further, the review revealed that on two occasions in the months before Abedi attacked, MI5 received intelligence, but its significance was not fully appreciated at the time and, in hindsight, was "highly relevant" to the planned attack.

The review also pointed out that Abedi was identified by MI5 as one of "a few dozen" people who needed further consideration.

However, the meeting to further consider these people, was due to take place on May 31, nine days after the attack.

Anderson’s review also revealed that there was no security service port alert against Abedi, so he was not questioned at the border when he returned to the U.K. from Libya four days before the attack.

Even more shockingly, the reviews also revealed that the two other attackers who had been on MI5's radar were Khuram Butt, the leader of the London Bridge and Borough Market attack, and Khalid Masood who targeted Westminster Bridge.

On June 3, a van hit pedestrians on London Bridge before three men got out and stabbed people in nearby Borough Market.

Before than, on March 22, a man in a hired car drove into pedestrians on Westminster Bridge then stabbed a police officer outside Parliament.

The review showed that two years earlier, Butt had been identified by MI5 and the police as someone who wanted to attack the U.K.

It said that he was still a "live subject of interest" who was under investigation at the time of the attack, though more for his intention to travel to Syria and for radicalising others.

He was also the main target of "Operation Hawthorn" - however, this was suspended twice because of a lack of resources after the Bataclan attack in Paris and the Westminster Bridge attack.

On the day Butt was attacked, Operation Hawthorn had resumed and had been running.

The former independent reviewer of terror legislation, Anderson said, "Despite elevated threat levels, the fundamentals are sound and the great majority of attacks continue to be thwarted. But the shock of these incidents has prompted intensive reflection and a commitment to significant change. In particular, MI5 and the police have identified the need to use data more effectively, to share knowledge more widely, to improve their own collaboration and to assess and investigate terrorist threats on a uniform basis, whatever the ideology that inspires them."

Meanwhile, in a statement to the Commons, Rudd said MI5 and the police had made 126 recommendations.

These recommendations reportedly included issues such as data sharing and analysis and how "closed subjects" should be managed, as well as a new approach to managing domestic extremism, particularly of right-wing groups.

Addressing MPs, she said, "We will shortly be announcing the budgets for policing for 2017/18, and I am clear that we must ensure counter terrorism policing has the resources needed to deal with the threats that we face.”

According to the Met Police, the number of dangerous, radicalised individuals was "a major issue,” with the Commissioner Cressida Dick saying her force needed "to make rapid progress in implementing the recommendations, many of which require new technology, better infrastructures and resources.”

Commenting on the revelations made in the review, the Mayor of Greater Manchester Andy Burnham said the report would be difficult reading for the people of his city.

He said, “It is clear that things could, and perhaps should, have been done differently and that wrong judgements have been made.”

He, however added that it should reassure the public to know MI5 were closing in on Abedi.

Burnham added that it would be much more worrying if nothing had been known about the attack.

Meanwhile, Chris Phillips, a former head of the National Counter Terrorism Security Office, said, "When you look back, within terrorism, you will always find some way that we could have stopped something. I always equate it to spinning plates. They've got hundreds and thousands of plates spinning at any given time. Someone has to make some risk-assessed decisions as to who is at the top of the pile to be watched."

Currently, the threat level for terrorism in the U.K. is severe, meaning an attack is highly likely.

On Tuesday, after senior ministers received a briefing from the head of the MI5 security service Andrew Parker,

Prime Minister Theresa May's spokesman said that Britain has thwarted nine plots in the last 12 months.

Detailing a presentation given to cabinet members on the current threat facing Britain, the spokesman said, “Mr Parker said that nine terrorist attacks had been prevented in the last year.”